Lab talk

Jun 22, 2007

Anti-reflective, super-hydrophobic surface

It is well known that structured surfaces such as lotus leaves can exhibit super-hydrophobic effects. However, in such structures the optical properties are usually sacrificed unless the feature size can be kept considerably below wavelength, otherwise the surfaces will result in unwanted scattering or diffraction. When optical properties are crucial, this limitation restricts the broader application of the advantages of a super-hydrophobic effect.

Recently a group of researchers at National Taiwan University (NTU) reported their work in Nanotechnology. The article discussed the trade-off between optical properties and super-hydrophobic effects. For example, we know that a moth's eyes are a type of nanostructure that can produce an anti-reflective surface effect. Is it possible to design a single nanostructure that possesses both super-hydrophobic and anti-reflective effects?

The group at NTU fabricated an inverted pyramid structure on silicon and coated it with Teflon. The structure was sub-wavelength and periodic, and was equivalent to a gradient-index medium for light. To reduce reflection, the profile of the structure was designed to allow the effective index to change gradually. The results showed that the design criterions for better anti-reflection performance and for higher hydrophobicity are the same, that is, increasing the depth increases both properties. Therefore the combination of these two effects in one nanostructure is possible and has the potential for many promising applications. The experimental research results obtained were consistent with the theoretical predictions.

The possible applications for this new technology include window coatings as well as windshield coatings, where high transparency is required and a self-cleaning feature is desirable. Also, applications that require long-term optical performance without the detrimental effect of contaminants, such as outdoor optical devices including LED lamps, display panels and solar cells, will be important.

About the author

Chih-Kung Lee is a professor and a founder of the NTU Nano-Bio MEMS Group at the Institute of Applied Mechanics, as well as a professor in the Department of Engineering Sciences and Ocean Engingeering, at NTU, Taiwan. Professor Lee is also a fellow of the Institute of Physics and ASME.